This invention relates to real-time processing of a raster-producing video signal by the use of statistics of the signal that corresponds to a local segment of the raster image.
The quality of display of an image in a video system can be degraded by extraneous video information in the form of, for example, shading variations that result from non-uniform illumination of the scene being imaged.
Such background noise of the scene adds a low frequency component to the video signal and increases the signal's dynamic range. If the video signal dynamic range exceeds the dynamic range of the display apparatus to which it is fed, low contrast image details can be lost.
One approach to removing the contribution of low-frequency noise to the dynamic range of video signal in order to permit the display of low contrast local scene details is to subject the video signal to a conventional local area contrast enhancement procedure that reduces the effect on dynamic range of such noise. Known procedures of this type enhance the contrast in a local area based on statistics derived from image characteristics in the area.
One example of localized adaptive enhancement is found in an article entitled "Local Adaptive Enhancement: A General Discussion In Fast Implementations" by E. C. Driscoll Et Al printed in the Proceedings of the 1983 Machine Processing Of Remotely Sensed Data Symposium. An approach to implementing a local area contrast enhancement algorithm in real time is found in "Real-Time Adaptive Contrast Enhancement," P. M. Narendra Et Al, IEEE TRANSACTIONS ON PATTERN ANALYSIS AND MACHINE INTELLIGENCE, Vol. PAMI-3, No. 6, November 1981.
In the Narendra apparatus, the statistical calculation of image area signal is performed by a separable two-dimensional low-pass filter. The filter consists of a conventional first filter section for filtering in the horizontal direction. The horizontal filter section is in series with a recursive first-order filter that filters in the vertical direction. Consequently, a spatial phase delay results between the image produced by the video signal and the signal output by the filters. As a result, the statistics at a point in the image scene area that is evaluated by the filter is based only on an asymmetric localized area that approaches but does not surround the point. Therefore, the calculated statistic does not account for image scene characteristics for the rest of the localized area around the point.
Therefore, to provide further enhancement of a video image that is processed in a local-area statistical manner, it would be advantageous to calculate the statistics for points in the scene from the variations in more symmetric, localized image areas that contain the points. This would result in calculation of statistical values more representative of image characteristics in the neighborhood of an image point, which, when used in a localized enhancement process, will produce a more desirable image.
It is therefore an objective of the present invention to provide an apparatus that performs a real-time, localized enhancement of a video image using image statistics for points in the video image that are derived from localized areas of the image surrounding the points.
It is a further object of the present invention to provide an improved separable, two-dimensional, higher-order filter for real-time processing of raster signals without introducing spatial phase delays between the data to be processed and the resulting processed data.
It is a still further object of the present invention to provide an improved apparatus performing a local-area contrast enhancement of video images.